Probing Surface Photovoltage Effect Using Photoassisted Secondary Electron Emission

Abstract

While the properties of surfaces and interfaces are crucial to modern devices, they are commonly difficult to explore since the signal from the bulk often masks the surface contribution. Here we introduce a methodology based on scanning electron microscopy (SEM) coupled with a pulsed laser source, which offers the capability to sense the topmost layer of materials, to study the surface photovoltage (SPV) related effects. This method relies on a pulsed optical laser to transiently induce an SPV and a continuous primary electron beam to produce secondary electron (SE) emission and monitor the change of the SE yield under laser illumination. We observe contrasting behaviors of the SPV-induced SE yield change on n-type and p-type semiconductors. We further study the dependence of the SPV-induced SE yield on the primary electron beam energy, the optical fluence, and the modulation frequency of the optical excitation, which reveal the details of the dynamics of the photocarriers in the presence of the surface built-in potential. This fast, contactless, and bias-free technique offers a convenient and robust platform to probe surface electronic phenomena, with great promise to probe nanoscale effects with a high spatial resolution. Our result further provides a basis to understand the contrast mechanisms of emerging time-resolved electron microscopic techniques, such as the scanning ultrafast electron microscopy.

Photoabsorption Imaging at Nanometer Scales Using Secondary Electron Analysis

“…Previous SUEM work attempted to model this coupling process but mainly focused on the transient behaviors and used only phenomenological rate conservation models. Recently Li et al measured the SE intensity change under pulsed laser illumination and a continuous electron beam . However, the transient photon flux in these works is much higher than that in our PAMELA-SEM experiments, so the dominant mechanism could be different.…”

“…Recently Li et al measured the SE intensity change under pulsed laser illumination and a continuous electron beam. 34 However, the transient photon flux in these works is much higher than that in our PAMELA-SEM experiments, so the dominant mechanism could be different. Here, the purpose of doing theoretical analysis is two-fold: (a) identify the mechanism underlying photoabsorption-induced SE emission signal; (b) provide qualitative explanation of the experimental trends.…”

Dynamical imaging of local photovoltage at semiconductor surface by photo-assisted ultrafast scanning electron microscopy

“…The state-of-the-art USEM literature focuses on the SE dynamics measured at the center of the laser spot and relates it to the SE emission yield [3,7,8,10]. In our setup, a spatial bipolar modulation of the SE signal around the laser spot appears, which we attribute to the breaking of the cylindrical symmetry with respect to surface normal axis by the side SE detector.…”

“…The study of dynamical Surface Photovoltages (SPV) provides information on the photophysics and charge transport in semiconductors. Recently, techniques of photo-assisted Time-Resolved Scanning Electron Microscopy (TR-SEM) were introduced to provide dynamical 2D mapping of SPV [1,2,3,4]. TR-SEM interpretation is based on the assumption that photoexcited spatial modulations in work function induce secondary electron (SE) contrast patterns.…”

Transient strain induced electronic structure modulation in a semiconducting polymer imaged by scanning ultrafast electron microscopy

“…Several mechanisms can contribute to the SUEM contrast [30], including accumulation of photocarriers [36], photo-induced modification of the surface potential [42,43], and photoinduced topographical changes [40]. By monitoring the space-time evolution of the secondary electron contrast, the relevant photo-physical process on the sample surface can be directly visualized with simultaneous high spatial and temporal resolutions.…”